This invention is directed generally to the field of antennas for communication systems, and more particularly to a novel active antenna system using patch/microstrip antenna elements, and more particularly still, to a novel lightning, corona, and low frequency static energy protection scheme for such an antenna system.
In base stations for most Cellular/PCS systems, where the antennas and cable are completely passive, lightning near strikes (or other corona discharges or high energy static) cause reliability concerns, since the antenna acts as a xe2x80x9cspongexe2x80x9d to the lightning (or corona/static discharge) energy, and channels the high voltage to the sensitive electronics. Of course, in the case of direct strikes, the antenna system is typically vaporized. However, for near strikes, where the local area around the antenna is saturated with high voltage field energy, protection of the base station electronics from this energy is warranted. These systems often employ xe2x80x9clightning arrestorxe2x80x9d systems, often simply high voltage-capable capacitors (high pass filters), that suppress the low frequency and DC (direct current) energy associated with the lightning. These arrestors are often simply attached in series with the cable to the antenna, near the antenna and/or near the base of tower (as shown in FIG. 1), via connectors, to the RF cable.
Additionally, even the presence of simple static build-up (DC energy), on the surface of the antenna elements, can achieve significant voltage to severely damage active components, not protected by the conventional lightning arrestor described above, i.e., a high voltage capacitor in series with the cable.
The above-referenced prior applications discloses a novel active amplifier system in which patch or microstrip type antenna elements are arranged in antenna arrays with each antenna element being provided with a low power amplifier chip closely adjacent the antenna element, or at least within the same housing or on the same circuit board as the antenna element.
For such xe2x80x9cactivexe2x80x9d antenna systems, which employ active electronics (amplifiers, transistors, phase shifters, . . . ) within the antenna structure, the use of the above-described conventional lightning arrestors will not protect the electronics. Such protection would require an arrestor system or device within the antenna itself, to arrest the low frequency and DC energy before it reaches any electronics. This proves difficult, since conventional arrestor devices are typically large (an inch or more in diameter) and costly. Additionally, the use of an arrestor of this type can adversely impact the performance of the electronics, since the capacitive properties of the arrestor adversely affects the circuit impedance.
The invention is described herein in connection with an aperture coupled microstrip patch antenna used in a base station sector antenna with active electronics; however, the invention is not so limited, but may be used in connection with patch antenna elements in other applications. Typically, the radiating microstrip patch is located on a dielectric superstrate and the DC voltage of the (metal) patch is floating with respect to zero potential or ground. If a static charge develops on the (metal) patch and discharges through the aperture to the microstrip feeder line, damage to, or failure of, the active electronics connected to the microstrip feeder line is possible. Since the antenna is operating with a single polarization, e.g., vertical polarization, any DC connection to the patch in the opposite polarization, e.g., horizontal polarization, does not affect the desired radiation pattern.
Therefore, to prevent static charge build up, the invention provides a narrow, high impedance conductive trace attached to the radiating patch in the orthogonal polarization (i.e., orthogonal to the patch polarization). These conductive traces are tied together with a vertical conductive trace along the axis of the array, which at a convenient location, is tied to an electrical ground.
In one embodiment, this grounding system of conductive traces is placed on the superstrate, so that the conductive traces do not disturb the base station""s radiation pattern or VSWR (voltage standing wave ratio). For the case of vertical polarization of the antenna elements, if the vertical traces which tie together the individual narrow static (horizontal) drain lines are too close to the radiating patch(es), the radiating pattern and VSWR can degrade. Therefore, the vertical trace is separated from the radiating patch. In one example of the invention, the vertical trace is roughly 0.45 xcexo (0.45 of a free space wavelength) away from the edge of the radiating patch.
If only one (vertical) trace is used to connect to the (horizontal) lines from the patch, generation of some undesirable asymmetry in the azimuth radiation pattern is possible. By designing a system of traces with symmetry about the center of the radiating patch, in one embodiment of the invention, mechanical symmetry is maintained, and accordingly, the azimuth radiation pattern remains symmetrical.
In an alternate embodiment of the invention, it is an objective to overlay the grounding system of conductive traces on the superstrate so that the conductive traces interact with the radiating patch to produce desirable effects in overall (azimuth) radiation pattern. Some of the desirable effects to the (azimuth) radiation pattern are: (a) to suppress backward radiation, and, (b) shaping of the pattern within the sector coverage, i.e., tailoring the pattern to roll off quicker past the sector edge.
Briefly, in accordance with the foregoing, an active antenna system having lightning, corona and low frequency static energy protection, comprises a plurality of patch antenna elements, a feed structure operatively interconnecting said plurality of patch antenna elements, and at least one conductive drain line coupled with each of said patch antenna elements, said drain lines being coupled together at a common ground connection point.